I ran into this problem too.. The issue turned out to be that the 'select' option does not seem to be available to the DwtPropertyEditor object. Instead, you have to create your own dialog box, without using DwtPropertyEditor & fill it with your own HTML:
Code:
this._view = new DwtComposite(this.getShell());
...
this._select = new DwtSelect(this._view);
Hope this helps!